Princeton University
COS 217: Introduction to Programming Systems

Assignment 1: A "De-Comment" Program

Purpose

The purpose of this assignment is to help you learn or review (1) the fundamentals of the C programming language, (2) the details of the "de-commenting" task of the C preprocessor, and (3) how to use the GNU/UNIX programming tools, especially bash, xemacs, and gcc.

Background

The C preprocessor is an important part of the C programming system. The C preprocessor performs two jobs. Its first job is to handle preprocessor directives (#define, #include, etc.) that reside in the given source code file. Its second job is to "de-comment" (that is, remove comments from) the given source code file. The first is the more difficult. Nevertheless, the second is substantial.

Your Task

Your task is to compose a C program named "decomment" that performs the de-commenting job of the C preprocessor.

Your program should be structured as a UNIX filter. That is, your program should read characters from standard input, and write characters to standard output (and possibly to standard error). Specifically, your program should read text (which presumably comprises a C program) from standard input, write that same text -- devoid of comments -- to standard output, and write error messages as appropriate to standard error. A typical command-line execution of your program might look like this:

decomment < somefile.c > somefilewithoutcomments.c 2> errormessages

The Details

Your program should:

Define "comment" as in the C89 standard. In particular, your program should consider text of the form (/* ... */) to be a comment. It should not consider text of the form (// ... ) to be a comment.
Allow a comment to span multiple lines. That is, your program should allow a comment to contain newline characters.
Not recognize nested comments. For example, your program should consider the text (/* abc /* def */ ghi */) to consist of the comment (/* abc /* def */) followed by the non-comment text ( ghi */).
Respect C string literals. In particular, your program should not consider text of the form (/* ... */) that occurs within a string literal ("...") to be a comment.
Respect C character literals. In particular, your program should not consider text of the form (/* ... */) that occurs within a character literal ('...') to be a comment.
Respect escaped characters within string literals. That is, your program should consider text of the form ("...\" ...") to be a valid string literal which happens to contain the double quote character.
Respect escaped characters within character literals. That is, your program should consider text of the form ('...\' ...') to be a valid character literal which happens to contain the quote character.

Your program should detect and report these three errors:

Newline character in a string literal. If your program reads a newline character before a string literal is terminated, it should terminate the string literal by writing a double quote to standard output, write the message "Line X: newline in string literal" to standard error, and proceed as if the literal had been terminated. "X" should be the number of the line which contains the offending string literal.
Newline character in a character literal. If your program reads a newline character before a character literal is terminated, it should terminate the character literal by writing a quote to standard output, write the message "Line X: newline in character literal" to standard error, and proceed as if the literal had been terminated. "X" should be the number of the line which contains the offending character literal.
Unterminated comment. If your program detects end-of-file before a comment is terminated, it should write the message "Line X: unterminated comment" to standard error. "X" should be the number of the line on which the unterminated comment begins.

You may assume that the last line of the input file ends with a newline character.

Your program should make sure that the text that it writes to standard output ends with a newline character. In particular, your program should write a newline character after an unterminated comment.

You should not make any assumptions about the maximum length of an input line.

Suggestion: The concept of deterministic finite state automaton, as described in the COS 126 course, is pertinent to this assignment. Designing your program as such can simplify its logic significantly.

Logistics

You should create your program on hats using xemacs and gcc.

Step 1: Create Source Code

Use xemacs to create source code in a file named decomment.c.

Step 2: Compile, Assemble, and Link

Use the gcc command with the -Wall, -ansi, and -pedantic options to preprocess, compile, assemble, and link your program.

Step 3: Execute

Execute your program multiple times on various input files that test all logical paths through your code.

To help you test your program, we have provided several files in hats directory /u/cos217/Assignment1:

sampledecomment is an executable version of a correct assignment solution. Your program should write exactly (character for character) the same data to standard output and standard error as does sampledecomment. You should test your program using commands similar to these:

sampledecomment < somefile.c > output1 2> errors1
decomment < somefile.c > output2 2> errors2
diff output1 output2
diff errors1 errors1
rm output1 errors1 output2 errors2
The UNIX diff command finds differences between two given files. The executions of the diff command shown above should produce no output. If the command "diff output1 output2" produces output, then sampledecomment and your program have written different characters to standard output. Similarly, if the command "diff errors1 errors2" produces output, then sampledecomment and your program have written different characters to standard error.

grade1 and grade1diff are bash scripts that will help you to automate your testing.
Several .txt files (that is, files whose names end with the ".txt") can serve as input files to your program.

Step 4: Create a readme File

Use xemacs to create a "readme" text file that contains:

Your name.
A description of whatever help (if any) you received from others while doing the assignment, and the names of any individuals with whom you collaborated, as prescribed by the course "Policies" web page.
(Optionally) An indication of how much time you spent doing the assignment.
(Optionally) Your assessment of the assignment: Did it help you to learn? What did it help you to learn? Do you have any suggestions for improvement? Etc.
(Optionally) Any information that will help us to grade your work in the most favorable light. In particular you should describe all known bugs.

Note that comments describing your code should not be in the readme file. Rather they should be integrated into the code itself.

Step 5: Submit

Submit your work electronically on hats via the command:

/u/cos217/bin/i686/submit 1 decomment.c readme

If the directory /u/cos217/bin/i686 is in your PATH environment variable, then you can abbreviate that command as:

submit 1 decomment.c readme

If you are using the bash shell and have copied files .bashrc and .bash_profile to your HOME directory, then directory /u/cos217/bin/i686 indeed is in your PATH environment variable. You can examine the value of your PATH environment variable by executing the command "printenv PATH".

Grading

We will grade your work on correctness and design. We will consider understandability to be an important aspect of good design. See the next section for guidelines concerning program understandability. To encourage good coding practices, we will compile using "gcc -Wall -ansi -pedantic" and take off points based on warning messages during compilation.

Program Understandability

An understandable program:

(1) Uses a consistent and appropriate indentation scheme. All statements that are nested within a compound, if, switch, while, for, or do...while statement should be indented. Most programmers use either a 3- or 4-space indentation scheme. Note that the xemacs editor can automatically apply a consistent indentation scheme to your program.

(2) Uses descriptive identifiers. The names of variables, constants, structures, types, and functions should indicate their purpose. Remember: C can handle identifiers of any length, and the first 31 characters are significant. We encourage you to prefix each variable name with characters that indicate its type. For example, the prefix "c" might indicate that the variable is of type "char," "i" might indicate "int," "pc" might mean "pointer to char," "ui" might mean "unsigned int," etc.

(3) Contains carefully worded comments. You should begin each program file with a comment that includes your name, the number of the assignment, and the name of the file. Each function -- especially the main function -- should begin with a comment that describes what the computer does when it executes that function. That comment should explicitly state what (if anything) the computer reads from standard input (or any other stream), and what (if anything) the computer writes to standard output and standard error (or any other stream). The function's comment should also describe what the computer does when it executes that function by explicitly referring to the function's parameters and return value.

Princeton University COS 217: Introduction to Programming Systems